Over recent years the interest in semiconductor nanowires has intensified. Nanowires are also referred to as nanowhiskers, nanorods and nanocolumns etc. For the purpose of this application the term nanowire is to be interpreted as a structure being in essentially in one-dimensional form and that is of nanometer dimensions in its width or diameter. Such structures are commonly also referred to as nanowhiskers, one-dimensional nano-elements, nanorods, nanocolumns, nanotubes, etc. Although these terms imply an elongated shape the nanowires may have e.g. a pyramidal shape. Usually nanowires are considered to have at least two dimensions not greater than 100 nm. However, nanowires having a diameter or width of about 1 μm can be formed. Controlling the one-dimensional growth on the nanometer scale offers unique opportunities for combining materials, manipulating properties, both mechanical and electromagnetical, and to design novel devices. One of the useful devices that can be made due to the controlled one-dimensional growth of nanowires is light emitting diodes (LED).
Epitaxial growth of III-V semiconductors on Si presents several difficulties such as lattice mismatch, differences in crystal structure (III-Vs have a polar zincblende or wurtzite structure whereas Si has a covalent diamond structure), a large difference in thermal expansion coefficient and the formation of so called anti-phase domains. Much work has been done on planar growth of III-V materials on Si using different approaches attempting to grow device quality structures, see e.g. S. F. Fang et al., Gallium-Arsenide and Other Compound Semiconductors on Silicon, Journal of Applied Physics 68, R31-R58 (1990) for a review.
It is recognized that a III-V semiconductor nanowires grown out from an Si substrate would probably overcome several of the above mentioned problems, due to the small cross-section of the conjunction between the nanowires and the Si substrate. In many systems nanowires grow in a <111> direction and most commonly in the [111]B direction. The growth of non-vertical wires from Si(111) substrates is commonly observed, see e.g. A. L. Roest et al., Position-controlled epitaxial III-V nanowires on silicon, Nanotechnology 17, (11), S271-S275 (2006), and poses a problem in terms of process control. There are four <111> directions available from the non-polar Si(111) substrate where only one is vertical as illustrated in FIG. 1a. In industrial applications this may hinder an efficient processing and lower the yield of useful components.